This invention relates to shoe manufacture and particularly to improvements in stiffening the shank region of a shoe insole.
It has been substantially universal practice for many decades to stiffen the shank region of a shoe by attaching a preformed steel, wood or like shank piece to the bottom of the insole. Depending on the particular shoe manufacturing process employed by the manufacturer, the shank may be applied before the insole is attached to any other portion of the shoe assembly or, in other instances, the shank may be attached to the insole as a subsequent step, after a substantial part of the shoe assembly already has been built up on a last.
Regardless of the manufacturing technique, the use of metal, wood or similar preformed shanks has caused numerous difficulties over the years. Substantial inventories of a wide variety of sizes and styles of shanks must be maintained. It is not uncommon for the shanks and shoes to be mismatched. Moreover, even when a proper shank piece is selected for a particular shoe, it is not uncommon for the shank piece, which often is simply stapled to the insole, to be misregistered on the insole. Preformed shanks must be placed on the insole in precisely the right location so that the curve of the shank will conform to the intended proper curve of the finished shoe. However, preformed shank pieces are difficult to place accurately and the operation requires significant operator skill. Failure to place the shank properly also may interfere with subsequent manufacturing operations, such as nailing the heel to the shoe.
Even when a preformed shank piece is assembled properly into a shoe difficulties can be presented when the shoe is worn. It is not uncommon for a steel shank piece to work its way loose from the repetitive flexing of the shoe, causing the shoe to develop a squeak. Another difficulty encountered with preformed steel shanks is "striking through", when the steel shank can be felt by the wearer. Efforts to overcome this have included the addition of sheets or layers of cushioning material (called a "tuck") interposed between the shank and the insole. Here again, this adds to the cost and only further complicates the manufacturing procedure.
Additional difficulties result from the manner in which insoles are manufactured. It is not uncommon for the insoles to be non-uniform in shape. Insoles usually are formed in a process in which the insole is molded in a molding press to a shape which is intended to fit the contour of the bottom of the last. Where the insoles are intended to be used with many identical lasts, variations in the insole shape, from insole to insole, can result in a variety of manufacturing problems as well as considerable difficulties in the quality of the resulting shoes. For example, improperly shaped insoles can result in loose top lines, wavy or wrinkly appearing uppers, poor lasting, misregistration of various shoe parts and the like. While subsequent attachment of a preformed, preshaped shank strip to a misformed or mismolded insole sometimes will tend to correct the insole shape, whether and the extent to which the insole will be so corrected depends in large measure on the care and skill of the operator who attaches the shank. Thus, while an overmolded or undermolded insole might be correctable by precision attachment of a preformed shank of proper curvature and shape, the practicalities of commercial production are such that such precision shank attachment cannot be relied on in most commercial settings. The present invention provides a means which assures that each and every insole will be precisely the same configuration both as to shape as well as to configuration of stiffener.
Prior patents assigned to the assignee of this invention, have described articles, techniques and devices for using initially flexible and deformable shank stiffener elements which, once applied to the bottom of an insole, then can be activated and cured to a hardened, stiff shank element. For example, materials useable for this purpose are described in U.S. Pat. Nos. 4,081,917, issued Apr. 4, 1978 and 4,162,583, issued Jul. 31, 1979. The present invention relates to further improvements in techniques and devices for forming insoles with shanks attached thereto and in which the shanks are attached to the insole before the insole is incorporated into a shoe assembly.